A known example of a rotation lock device is a reclining device for a vehicle seat that is shown by, for example, Japanese Laid-Open Patent Publication No. 2001-157615. In the device disclosed, a seat back is connected to a seat cushion via a reclining device, so that a tilting angle of the seat back can be adjusted.
In the reclining device, a disc-shaped ratchet bonded to the seat back and a disc-shaped guide bonded to the seat cushion are assembled together in a manner that their respective disc surfaces contact each other, so as to be relatively rotated. The ratchet has an inner toothed portion that is formed along an outer periphery thereof. The inner toothed portion has inner teeth that are formed in an inner peripheral surface thereof. The inner toothed portion is formed by cylindrically protruding in an axial direction of the ratchet, so that the inner teeth are exposed to an inner space defined by the two discs in a condition that the ratchet is combined to the guide. Conversely, the guide has a depressed guide portion formed in the inner disc surface that faces to the ratchet, so that a plurality of pawls having outer teeth are received therein. The guide portion guides each of the pawls, so that the pawls can move only in a radial direction of the guide. A cam that is rotated by an unlock lever for reclining operation is disposed at a central portion of the guide. The cam is normally biased so as to push the pawls radially outwardly, so as to maintain a condition in which the outer teeth of the pawls mesh with the inner teeth of the ratchet. As a result, relative rotation of the ratchet and the guide is restricted, so that the reclining device is maintained in a locked condition. When operation of the unlock lever is performed, the cam stops pushing the pawls outwardly, so as to release a meshing condition between the outer teeth of the pawls and the inner teeth of the ratchet. As a result, the relative rotation of the ratchet and the guide is allowed, so that the reclining device can be switched to an unlocked condition. When the operation of the unlock lever is stopped, the cam is rotated by a biasing force. As a result, the pawls are pushed radially outwardly, so that the reclining device can be automatically returned to the locked condition.
The reclining device described above has a free zone in which the reclining device can be maintained in the unlocked condition even if the operation of the unlock lever is stopped. The free zone is formed in a rotational region which is not used when the seat is in a use condition, for example, a rotational region in which the seat back is forwardly tilted with respect to the seat cushion. In particular, the free zone is formed by partially providing a non-toothed raised flat portion on the inner peripheral surface of the inner toothed portion of the ratchet, which portion is formed so as to be protruded radially inwardly beyond tips of the inner teeth. That is, in the region in which the raised flat portion is formed, either one of the pawls, when pushed radially outwardly, may contact the raised flat portion that is protruded inwardly beyond the inner teeth, so that rotation of the cam to push the other pawls outwardly is restricted. As a result, the outer teeth of the pawls and the inner teeth of the ratchet are prohibited from meshing with each other. Thus, in the rotation region in which this raised flat portion is formed, it is possible to change an inclination angle of the seat back without maintaining the operating condition of the unlock lever. As a result, it is possible to easily perform, for example, folding operation of the seat back onto the seat cushion.